Sagan's Response

A rebuttal of Tipler's
Argument, put forward by Carl Sagan and William
Newman of Cornell University.1
They first pointed out that Tipler had underestimated the number
of von Neumann probes there ought
to be. With exponential growth, a single self-replicating probe could be
expected to convert the entire mass of the Galaxy into copies of
itself within 2 million years. Any species intelligent enough to build such
a probe, Sagan and Newman argued, would also be intelligent enough to realize
the danger of it and so would not embark upon the project in the first place.
In the event of a von Neumann probe being released either accidentally or
maliciously, it would be a prime duty of other, responsible civilizations,
said Sagan and Newman, to stamp out the "infection" before it could spread.